I don't mean to go into full fanboy mode but some of this needs to be addressed.

I love both Witcher 3 and Horizon, for the record.
There's tons of weapons, but it's a lot of "here's the same one with one of its 5 stats better by 8%". Not even to mention the random garbage swords drops from random grunts. Why would they even make them lootable.
This isn't a loot game in the sense of like Diablo or Destiny. You're not checking every dead body and chest for the chance to find an awesome piece of gear, you're looting to get the money and mats to craft the awesome gear.
There's a fuckton of loot, but most of it is utterly useless. You quickly reach the point where you won't even bother looting stuff, because in 99% of cases it's some useless piece of kitchenware or whatever.

Why would you stop looting? It's free money and materials. The mats are probably the most important part because the ingots/leather/monster parts allow one to craft the Witcher sets. The Witcher sets are the actual stuff you should be wearing and there is a set to accommodate every general build. There isn't an excuse to be wearing garbage or using basic swords. (6 different sets with the DLC between Cat, Bear, Griffin, Wolf, Viper and Manticore, you can jump from one set to the next as you level up). Every time I've watched a Let's Play and they're wearing some multicolored burlap sack and they're crying about the "shitty loot" I want to rip my hair out.
There's tons of sidequests, but they fall into a handful of extremely repetitive categories.
Witcher's strength is in the writing, characters and worldbuilding. If you're not invested into those, you're missing out on what makes the game great.
The sidequests and Hunts are basically the "monster of the week" episodes of a TV series. If you really expected each sidequest to provide a new
gameplay experience I think you have some skewed expectations of what the game is. They're providing another peak into the lives and people of this very deep and fleshed out (relative to most video games, anyway) universe.
There's the occasional cool environment, but most of it looks the same.
It's pretty clear what the intention was here. They wanted to create a "real", lived in place and they succeeded in a way no other open-world game ever has IMO. To a point they needed to be stick to their source material and the world established in from the books and previous games. There was never going to be a lava level and a ice level and sky level or super videogamey locales. The Northern Realms is an established location with its own history and the story being told primarily takes place during a certain time of year.
It would be like creating a Game of Thrones game based around just King's Landing and the surrounding countryside and being disappointed there is not desert, jungle or ice areas.
Horizon had the benefit of 1) being a new IP and 2) the area the game takes place in is super condensed geographically in relation to the real life equivalent. It allowed for a bunch of different biomes and terrain which really wouldn't have been possible given the size of the area being portrayed. Plus (
HORIZON ZERO DAWN SPOILERS)
spoiler (click to show/hide)
There aren't tropical rainforests in Southwest United States.

The sci fi setting allowed them to play around with that kind of stuff and diversify the play areas.